The present disclosure relates to earth retention systems including retaining walls built from cellular confinement systems, also known as geocells. In particular, such retaining walls are especially resistant to dynamic loads, such as shock waves related to seismic activity from earthquakes. The present disclosure also relates to the components of such walls and methods for making and using such retaining walls.
A cellular confinement system (CCS) is an array of containment cells resembling a “honeycomb” structure that is usually filled with cohesionless soil, sand, gravel, or any other type of aggregate. Also known as geocells, CCSs are used in applications to prevent erosion or provide lateral support, such as gravity retaining walls for soil, alternatives for sandbag walls, and for roadway and railway foundations. The infill and the geocell are coupled via friction and interlocking mechanisms. CCSs differ from geogrids or geotextiles in that geogrids/geotextiles are generally flat (i.e., two-dimensional) and used as planar reinforcement, whereas CCSs are three-dimensional structures with internal force vectors acting within each cell against all the walls. In addition, stress transfer in geogrids/geotextiles is much more sensitive to the infill type and installation quality. Geocells, on the other hand, can tolerate more damage due to its three-dimensional structure.
CCSs are commercially available, such as the Geoweb® earth retention system, from Presto Products Company, a popular CCS. Presto utilizes polyethylene (PE) as the material of choice when fabricating geocells. Polyethylene is low cost and has very good chemical resistance. However, relative to other polymeric materials used in soil reinforcement (e.g., polyester, polyvinyl alcohol), polyethylene has low stiffness, low strength, high creep, and high coefficient of thermal expansion. In particular, PE's long term stiffness is about 20%-25% that of its original stiffness. This decreases further when it is subjected to elevated temperatures.
With regards to the aggregate material placed in a CCS, one such material is soil. Soil is any material found in the earth at a locality, which may comprise of naturally derived solids including organic matter, liquids (primarily water), fine to coarse-grained rocks and minerals, and gases (air). The liquids and gases occupy the voids between the solid particles. The packing of soil is known as densification and is achieved during construction by compaction. Compaction is the process in which high load is temporarily applied to the soil by mechanical means such as a roller. When soil is compacted, the solid particles are forced closer together, eliminating any volume in the voids that is occupied by air. Dense soil is rather strong under compression, but has little to no strength under tension. When granular soil is compacted to a dense state, as is required in proper construction, it will reach its peak shear strength under compressive stresses at rather low strain—usually at 1 to 3% strain. However, at larger strains, it will quickly reach lower shear strength than its peak as it undergoes through a strain-softening phenomenon.
The compressive strength and availability of soil makes it desirable as filler for CCSs. When soil is reinforced, such as with a geogrid, a composite structure is formed that is strong under both compression and tension, compared to the original soil.
A CCS contributes to soil strength in several ways. First, the cells of the CCS surround and confine the soil. When a compressive stress is applied to the surface of a geocell infilled with soil, the lateral stress exerted by soil outside the geocell on the cell walls increases as well. The increased soil lateral pressure on the cell walls result in the walls exerting compressive lateral pressure on the soil confined within the cell walls. The increase in the lateral, confining, stress can be as large as the increase in the applied compressive stress. Because the strength of the infill material depends on the lateral stress, an increase in the lateral stress increases the strength of the infill material. In fact, using a stiff cell wall to confine the infill would create a situation where failure of the confined infill will occur only when the solid particles crush or the cell walls undergo large deformation or rupture. As a result, the confined infill exhibits a greater lateral strength for a given depth, compared to unconfined infill.
This principle can be illustrated by soil at various depths. Granular soil at the top of a surface has zero strength at zero confinement so that even weak forces (such as wind) can move the soil particles. Driving a stake into the ground, shearing the soil under compression initially requires little effort. However, trying to drive a stake into the ground gets more difficult the deeper one tries to drive it. The deeper soil is confined because it cannot move laterally and allow shear failure to develop.
In addition, a CCS confines soil, whereas a geogrid does not. As the density of soil in a cell increases, its strength and its stiffness increase dramatically. A thoroughly filled CCS with adequately compacted soil forms a composite structure that, at high enough densities, is analogous to steel reinforced concrete.
To strengthen the interaction between the geocell and the infill, their interface should be rough to maximize frictional resistance and increase the bonding between the two materials. The geocell should also be stiff and creep resistant enough that it will not relax. Relaxation and creep may allow the confined infill to move lateral, resulting in loss of compressive strength.
Unfortunately, creep and relaxation will occur in polyethylene under relatively small loads, such as 10-25% % of its short-term ultimate strength when considering the typical life span of a CCS. Geocells made from polyethylene thus do not perform well over long periods of time because the stress, which increases the strength of the infill, relaxes. Polyethylene is also of limited stiffness (lower than 1 GPa at ambient at 150% per minute strain rate, lower than 600 MPa at temperatures of 40-60° C. at 150% per minute strain rate) and has a high tendency to creep.
Accordingly, it would be beneficial to provide a structure that uses the compressive strength of soil, the tensile strength, stiffness, and dimensional stability of a CCS, and is resistant to dynamic loading.